The present invention relates to a method for etching an organic film, a method for fabricating a semiconductor device and a pattern formation method.
For the purpose of increasing the operation speed and lowering the consumption power of semiconductor devices, decrease of the dielectric constant of an interlayer insulating film included in a multi-level interconnect structure is recently regarded as significant. In particular, an organic film with a small dielectric constant can be easily formed by spin coating and curing, and hence is regarded as a very promising interlayer insulating film of the next generation. A well known example of the organic film with a small dielectric constant is an organic film including an aromatic polymer as a base.
In order to fabricate a device with a refined design rule of a gate length of 0.18 xcexcm or less, a fine line processing technique of approximately 0.25 xcexcm or less is necessary, and the design rule is considered to be more and more refined in the future. An organic film is generally patterned by plasma etching, but a fine pattern of 0.25 xcexcm or less is very difficult to form from an organic film.
Known examples of the plasma etching employed for an organic film are a process using an etching gas including a N2 gas and a H2 gas as principal constituents (reported by M. Fukusawa, T. Hasegawa, S. Hirano and S. Kadomura in xe2x80x9cProc. Symp. Dry Processxe2x80x9d, p. 175 (1998)) and a process using an etching gas including a NH3 gas as a principal constituent (reported by M. Fukusawa, T. Tatsumi, T. Hasegawa, S. Hirano, K. Miyata and S. Kadomura in xe2x80x9cProc. Symp. Dry Processxe2x80x9d, p. 221 (1999)).
One of conventional etching methods will now be described as Conventional Example 1 referring to the result obtained by etching an organic film with a magnetic neutral loop discharge (NLD) plasma etching system manufactured by Ulvac Corporation (xe2x80x9cSiO2 Etching in magnetic neutral loop discharge plasmaxe2x80x9d, W. Chen, M. Itoh, T. Hayashi and T. Uchida, J. Vac. Sci. Technol., A16 (1998) 1594).
In Conventional Example 1, an organic film is etched by using an etching gas including a N2 gas and a H2 gas as principal constituents. The present inventors have carried out the etching process of Conventional Example 1 under the following conditions:
Plasma etching system: NLD plasma etching system
Volume flow ratio per minute in standard condition of etching gas:
N2:H2=50 ml:50 ml
Antenna power: 1000 W (13.56 MHz)
Bias power: 200 W (2 MHz)
Pressure: 0.4 Pa
Substrate cooling temperature: 0xc2x0 C.
Etching time: 180 seconds
FIGS. 13A through 13D are cross-sectional SEM photographs of holes formed under the aforementioned etching conditions in organic films, and the holes of FIGS. 13A through 13D have diameters of 0.16 xcexcm, 0.18 xcexcm, 0.24 xcexcm and 0.40 xcexcm, respectively. In FIGS. 13A through 13D, a reference numeral 101 denotes a silicon substrate, a reference numeral 102 denotes an organic film to be etched, and a reference numeral 103 denotes a mask pattern of a silicon oxide film used as a mask in etching the organic film 102. The organic film 102 has a thickness of approximately 1.02 xcexcm, and the mask pattern 103 has a thickness of approximately 240 nm.
Conventional Example 1 is described as a process using the etching gas including, as principal constituents, a N2 gas and a H2 gas, and another method for etching an organic film is proposed as a process using an etching gas including, as principal constituents, an O2 gas, a N2 gas and a C2H4 gas (Genexh Rajagopalan, et al.; Abstra. The 1999 Joint International Meeting of ECS, Hawaii, October, 702 (1999)).
Now, a method for fabricating a semiconductor device according to Conventional Example 2 will be described with reference to FIGS. 14A and 14B.
FIGS. 14A and 14B show states where an organic film 105 formed on a semiconductor substrate 104 is subjected to plasma etching by using a mask pattern 106 of, for example, a silicon oxide film formed on the organic film 105. FIG. 14A shows a state in the middle of the plasma etching and FIG. 14B shows a state after completing the plasma etching. In FIGS. 14A and 14B, a reference numeral 107 denotes a first recess having a small diameter and a reference numeral 108 denotes a second recess having a comparatively large diameter. Although not shown in the drawings, a metal material film is formed over the mask pattern 106 so as to fill the first recess 107 and the second recess 108, and a portion of the metal material film formed on the mask pattern 106 is removed by, for example, chemical mechanical polishing (CMP), so as to form a connection plug or a metal interconnect from the metal material film.
As is shown in FIG. 14A, the etching rate of the first recess 107 having a small diameter is lower than the etching rate of the second recess 108 having a comparatively large diameter.
Also, as is shown in FIG. 14B, the etching time required for completing etching the first recess 107 is generally calculated on the basis of the etching rate of the first recess 107, and over-etching of several tens % is generally conducted in addition to the calculated etching time so as to completely remove the organic film 105 remaining on the semiconductor substrate 104 within the recess.
As methods of forming a mask pattern through dry development, a top surface imaging (TSI) process, a three-layer resist process and the like are known.
In the top surface imaging process, a surface of an organic film resulting from pattern exposure is subjected to silylation, so as to selectively form a silylated layer on an exposed or unexposed portion of the organic film. Then, the organic film is subjected to dry development using the silylated layer as a mask, so as to form a mask pattern.
In the three-layer resist process, after an organic film and a silicon oxide film are successively formed on a semiconductor substrate, a thin resist pattern is formed on the silicon oxide film. Then, the silicon oxide film is subjected to plasma etching by using the resist pattern as a mask, so as to form an oxide film pattern by transferring the resist pattern onto the silicon oxide film. Next, the organic film is subjected to dry development (plasma etching) by using the oxide film pattern. Thus, a fine organic film pattern having a high aspect ratio is formed from the organic film.
Furthermore, an etch target film formed on the semiconductor substrate is etched by using a two-layer mask pattern consisting of the oxide film pattern and the organic film pattern. In this manner, a fine pattern that cannot be resolved by a single layer resist can be formed in the etch target film.
The present inventors have carried out the three-layer resist process, as a mask pattern formation method for Conventional Example 3, by using an etching gas including an O2 gas under the following etching conditions:
Plasma etching system: NLD plasma etching system
Flow rate per minute in standard condition of etching gas: O2=90 ml
Antenna power: 1000 W (13.56 MHz)
Bias power: 400 W (2 MHz)
Pressure: 0.133 Pa
Substrate cooling temperature: 0xc2x0 C.
Etching time: 4 minutes
FIGS. 16A and 16B are cross-sectional SEM photographs of holes formed in an organic film pattern by the pattern formation method for Conventional Example 3, and the holes of FIGS. 16A and 16B have diameters of 0.18 xcexcm and 0.4 xcexcm, respectively. In FIGS. 16A and 16B, a reference numeral 111 denotes a silicon substrate, a reference numeral 110 denotes an organic film pattern formed from an organic film, and a reference numeral 109 denotes an oxide film pattern formed from a silicon oxide film. A resist pattern present on the oxide film pattern 109 is eliminated during the formation of the organic film pattern 110 through the dry development, and hence, an etch target film deposited on the silicon substrate 111 is etched by using the two-layer mask pattern consisting of the oxide film pattern 109 and the organic film pattern 110.
FIG. 15 is a diagram of the RIE lag characteristic of the method for etching an organic film of Conventional Example 1. The RIE lag is a phenomenon that the etching rate is lowered when the aspect ratio of a recess to be etched is increased, which means that the etching rate is lowered as the processing dimension (the dimension of an opening to be formed) is smaller in etch target films having the same thickness.
FIG. 15 shows the relationship between the diameter of a hole and the etching depth obtained when holes having a diameter of 0.18 xcexcm through 0.4 xcexcm are formed in an organic film by conducting etching for 180 seconds by using an etching gas of a mixed gas including a N2 gas and a H2 gas in a ratio in the flow rate (ml) per minute in the standard condition, namely, N2:H2, of 0:100, 30:70, 50:50, 70:30 or 100:0. As is understood from FIG. 15, the typical RIE lag characteristic is observed in any of the ratios.
When the RIE lag characteristic is increased, a process margin such as allowance in etching amount is reduced in forming a fine pattern. Therefore, when holes with different diameters or interconnect grooves with different widths are formed together, excessive or insufficient etching is caused in the respective holes or interconnect grooves, which causes variation in the etching amount in underlying films. As a result, the reliability of the semiconductor device is lowered.
Moreover, since large over-etching is required for compensating the insufficient etching, variation in the dimension caused in transferring a pattern is increased. As a result, it is very difficult to highly precisely form a fine pattern.
As described above, in the conventional method for fabricating a semiconductor device, the over-etching of several tens % is generally conducted in addition to the calculated etching time. Therefore, when the etch point reaches the semiconductor substrate 104 in the second recess 108 (having a comparatively large diameter) with the high etching rate, the etch point has not reached the semiconductor substrate 104 in the first recess 107 (having a small diameter) with the low etching rate.
Furthermore, as described above, the etching time is determined on the basis of the time required for completing etching the first recess 107.
Accordingly, excessive over-etching disadvantageously occurs in the bottom of the second recess 108.
Moreover, in the case where the etching time is insufficient, although the second recess 108 is sufficiently etched, the first recess 107 is disadvantageously insufficiently etched.
Accordingly, in the case where holes having different diameters or interconnect grooves having different widths are formed together, the excessive or insufficient etching is caused, which causes variation in the etching amount in underlying films. As a result, the reliability of the semiconductor device is lowered.
In Conventional Example 3, the dry development is carried out on the organic film through the plasma etching using the etching gas including an O2 gas as a principal constituent. Therefore, as is shown in FIGS. 16A and 16B, the hole formed in the organic film pattern 110 has a diameter larger than the diameter of an opening of the oxide film pattern 109, and the hole formed in the organic film pattern 110 has a bowing cross-section. When the etch target film is etched by using the organic film pattern 110 with the hole having such a bowing cross-section, it is difficult to highly precisely conduct the etching.
Therefore, in a method proposed for suppressing the hole of the organic film pattern 110 from having a bowing cross-section, the dry development is carried out on the organic film with the actual substrate temperature kept at a temperature below the freezing point by setting the substrate cooling temperature (refrigerant temperature) to 20xc2x0 C. through 50xc2x0 C. below zero.
In order to attain such a low temperature, however, excessive cost and a large-scaled system are required, and hence, there arise problems of increase of the system cost and decrease of the system stability. Therefore, it is not preferable that the substrate cooling temperature is set to 20xc2x0 C. through 50xc2x0 C. below zero.
As described so far, the problem that the hole formed in the organic film pattern 110 has a diameter larger than the diameter of the opening of the oxide film pattern 109 and the problem that the hole formed in the organic film pattern 110 has a bowing cross-section have not been solved yet.
Needless to say, the problems occurring in the three-layer resist process occur in the top surface imaging process.
In consideration of the aforementioned conventional problems, a first object of the invention is stably and uniformly etching an organic film by minimizing a RIE lag characteristic so as to avoid excessive or insufficient etching even when holes with different diameters or interconnect grooves with different widths are formed together.
A second object of the invention is, in fabricating a semiconductor device including holes with different diameters or interconnect grooves with different widths, improving the reliability of the semiconductor device by avoiding excessive or insufficient etching so as to suppress variation in the etching amount in underlying films.
A third object of the invention is, in forming an organic film pattern through dry development, highly precisely forming a mask pattern with a large process margin by preventing an opening of the organic film pattern from having a dimension larger than the dimension of an opening of a mask used for forming the organic film pattern and by forming an opening with a vertical cross-section or a cross-section tapered toward the bottom (hereinafter referred to as a forward taper cross-section) in the organic film pattern.
In order to achieve the first object, the method for etching an organic film of this invention comprises a step of etching an organic film by using plasma generated from an etching gas containing a first gas including a straight chain saturated hydrocarbon and a second gas including a nitrogen component.
In the present method for etching an organic film, an organic film is etched by using plasma generated from the mixed gas containing the gas including a hydrocarbon and the gas including a nitrogen component. Therefore, a deposition film is formed on an etch target surface, and owing to the deposition film, an ion assisted reaction is caused on the bottom of a recess substantially without depending upon the aspect ratio. Accordingly, a constant etching rate can be obtained without depending upon the aspect ratio, namely, the diameter of the recess.
In particular, since a straight chain saturated hydrocarbon is used as the hydrocarbon in the method for etching an organic film, a recess with a vertical or forward taper cross-section can be formed in the organic film with a very small RIE lag characteristic.
Accordingly, even when a fine pattern is to be formed, a process margin such as allowance in etching amount can be large, and even when holes with different diameters or interconnect grooves with different widths are to be formed together, excessive or insufficient etching can be avoided, so that underlying films can be substantially uniformly etched.
In the method for etching an organic film, the etching gas preferably further contains a gas including a compound including carbon, nitrogen and hydrogen.
When the gas including a compound including carbon, nitrogen and hydrogen is thus mixed with the mixed gas containing the gas including the straight chain saturated hydrocarbon and the gas including the nitrogen component, a recess to be formed can attain a forward taper cross-section with keeping the very small RIE lag characteristic. Furthermore, by adjusting the mixing ratios of the mixed gas containing the gas including the straight chain saturated hydrocarbon and the gas including the nitrogen component and the gas including the compound including carbon, nitrogen and hydrogen, the angle of the forward taper cross-section and the RIE lag characteristic can be controlled. The compound including carbon, nitrogen and hydrogen may be methylamine.
In the method for etching an organic film, the first gas is preferably a methane gas and the second gas is preferably a nitrogen gas.
In this manner, a recess with a vertical or forward taper cross-section can be definitely formed in the organic film with a very small RIE lag characteristic.
In the method for etching an organic film, the etching gas preferably further contains a rare gas.
In this manner, a deposition formed on the inner walls of a reaction chamber used for the etching can be reduced.
The method for fabricating a semiconductor device of this invention comprises the steps of forming an organic film on a semiconductor substrate; forming, on the organic film, a mask pattern including an inorganic compound as a principal constituent; and forming a recess in the organic film by selectively etching the organic film by using the mask pattern and by using plasma generated from an etching gas containing a first gas including a straight chain saturated hydrocarbon and a second gas including a nitrogen component.
In the present method for fabricating a semiconductor device, an organic film is etched by using plasma generated from the mixed gas containing the gas including hydrocarbon and the gas including a nitrogen component, namely, a semiconductor device is fabricated by the present method for etching an organic film. Therefore, a recess with a vertical or forward taper cross-section can be formed in the organic film with a very small RIE lag characteristic.
Accordingly, even when a fine pattern is to be formed, a process margin such as allowance in etching amount can be large, and even when holes with different diameters or interconnect grooves with different widths are to be formed together, excessive or insufficient etching can be avoided, so as to substantially uniformly etch underlying films. As a result, the reliability of the semiconductor device can be improved.
In the method for fabricating a semiconductor device, the etching gas preferably further contains a gas including a compound including carbon, nitrogen and hydrogen.
In this manner, a recess to be formed can attain a forward taper cross-section with keeping the very small RIE lag characteristic, and the angle of the forward taper cross-section and the RIE lag characteristic can be controlled by adjusting the mixing ratio, in the etching gas, of the gas including the compound including carbon, nitrogen and hydrogen. The compound including carbon, nitrogen and hydrogen may be methylamine.
In the method for fabricating a semiconductor device, the recess preferably includes a via hole and an interconnect groove formed above the via hole and is filled with a metal material film by a dual damascene method.
In this manner, the recess including the via hole and the interconnect groove formed above the via hole, the via hole in particular, can attain a forward taper cross-section with keeping the very small RIE lag characteristic. As a result, good electric connection can be attained between a connection plug and a lower metal interconnect disposed below the connection plug formed by the dual damascene method. Thus, the electric characteristic of a multi-level interconnect structure formed by the dual damascene method can be improved.
In the method for fabricating a semiconductor device, the first gas is preferably a methane gas and the second gas is preferably a nitrogen gas.
In this manner, a recess with a vertical or forward taper cross-section can be definitely formed in the organic film with a very small RIE lag characteristic.
In the method for fabricating a semiconductor device, the etching gas preferably further contains a rare gas.
In this manner, a deposition formed on the inner walls of a reaction chamber used for the etching can be reduced.
The pattern formation method for this invention comprises the steps of forming an organic film on a substrate; forming, on the organic film, a mask layer including an inorganic component; and forming an organic film pattern from the organic film by selectively etching the organic film by using the mask layer and by using plasma generated from an etching gas containing a first gas including a straight chain saturated hydrocarbon and a second gas including a nitrogen component.
In the present pattern formation method, the organic film pattern is formed by conducting selective etching on the organic film by using plasma generated from the etching gas containing the gas including a straight chain saturated hydrocarbon and the gas including a nitrogen component, namely, the organic film pattern is formed by the method for etching an organic film of this invention. Therefore, an opening formed in the organic film pattern can be prevented from having a larger dimension than an opening of the mask layer, and an opening with a vertical or forward taper cross-section can be formed in the organic film pattern with a very small RIE lag characteristic. Accordingly, a mask pattern can be highly precisely formed with a large process margin.
In the pattern formation method, the etching gas preferably further contains a gas including a compound including carbon, nitrogen and hydrogen.
In this manner, with keeping the very small RIE lag characteristic, the opening of the organic film pattern can attain a forward taper cross-section, and the angle of the forward taper cross-section and the RIE lag characteristic can be controlled by adjusting the mixing ratio, in the etching gas, of the gas containing the compound including carbon, nitrogen and hydrogen. The compound including carbon, nitrogen and hydrogen may be methylamine.
In the pattern formation method, the first gas is preferably a methane gas and the second gas is preferably a nitrogen gas.
In this manner, a recess with a vertical or forward taper cross-section can be definitely formed in the organic film pattern with a very small RIE lag characteristic.
In the pattern formation method, the etching gas preferably further contains a rare gas.
In this manner, a deposition formed on the inner walls of a reaction chamber used for the etching can be reduced.
In the pattern formation method, the mask layer is preferably a silylated layer.
In this manner, an opening with a vertical or forward taper cross-section can be formed in the organic film pattern with a very small RIE lag characteristic by the top surface imaging process.